Known radiotelephony systems such as G.S.M are dedicated essentially to voice communications. They use a channel comprising two symmetric channels, namely a downlink channel (from an earth base station to a mobile station) and an uplink channel (from the mobile station to the base station).
Systems under development are also based on such a structure. Thus, the UMTS standard defined by the ETSI allows a symmetric distribution between the downlink channel and the uplink channel.
It is also proposed to complete the radiotelephony system by adding at least one supplementary channel to the principal channel, in the down direction only and dedicated to transmission of high speed data such as files transmitted on the Internet network.
In order to simplify the situation, the following describes the disadvantages of prior art with relation to this particular application mentioned above. However, it is clear that this discussion could be transposed to other radiotelephony systems including a symmetric two-directional principal channel and at least one supplementary channel.
Remember that the HS-DPA (High-Speed Downlink Packet Access) supplementary link is a high speed downlink associated with a UMTS principal link. It is intended to increase the downlink speed to provide services requiring high speed (multimedia, video streaming, etc.).
As shown in FIG. 1, the UMTS principal link has a structure organized in N, N+1 frames each comprising 15 slots (time intervals) S1 to S15. As shown in FIG. 2, the HS-DPA supplementary link has a structure organized into N, N+1 frames each comprising up to 5 sub-frames SF1 to SF5. Moreover, each slot or sub-frame comprises a set of symbols, and each symbol comprises a set of signal units (chips).
For the physical layer of the HS-DPA supplementary link, two technical solutions are suggested:                a spectrum spreading system conforming with the UMTS system;        a system based on an OFDM multicarrier modulation.        
With the first solution, an UMTS HS-DPA supplementary link is obtained that is inherent to the UMTS system. Therefore, it can benefit from all the techniques already used by the UMTS principal link, such as channel estimation, control of power and clocks, and particularly synchronization made with the CPICH signal specified in the UMTS standard.
With the second solution, the result is an OFDM HS-DPA supplementary link that uses a modulation different from that used in the UMTS system (spectrum spreading, CDMA). Consequently, it cannot use all techniques used in the UMTS system. Therefore, it must use specific techniques so as to accomplish the same functions. Nevertheless, some adaptation to the context can facilitate setting up and maintaining communication with the OFDM link.
An OFDM sub-frame of the OFDM HS-DPA supplementary link and a UMTS sub-frame of the UMTS HS-DPA supplementary link have the same duration, namely 2 ms, and are interchangeable. The OFDM HS-DPA and UMTS HS-DPA supplementary links may each use a distinct clock (or time base) but their architectures are such that the beginning and end of the OFDM and UMTS sub-frames are identical.
Synchronization in time is one of the key elements in setting up a communication. This synchronization is broken down into several “layers” due to the nature of the radiomobile cellular communication system that defines the two entities; sub-frame and frame (see FIG. 2). Thus, for the HS-DPA supplementary link, this synchronization is divided into several steps:                synchronization at chip level, that consists of finding the position of symbols (and therefore of chips included in these symbols) depending on the clock used;        synchronization at sub-frame level, that consists of finding the beginning of sub-frames;        synchronization at frame level, that consists of finding the beginning of each frame.        
The UMTS HS-DPA supplementary link may be synchronized relatively easily. Since the UMTS HS-DPA supplementary link is intimately linked to the UMTS system, it can be synchronized directly based on the UMTS principal link. Thus, initial synchronization of the UMTS HS-DPA supplementary link at the chip may be done by a temporal self-correlation on a specific synchronization signal (PSCH) provided in the UMTS. After acquiring this synchronization at chip level, the UMTS HS-DPA supplementary link can be synchronized at sub-frame level by making a search for the beginning of the UMTS slots (knowing that each sub-frame contains a predetermined number of UMTS slots, for example 3). This search is done using the PSCH signal. This signal is in the form of a packet of 256 identical chips sent at the beginning of each slot. Finally, the frame of the UMTS HS-DPA supplementary link is synchronized using the SSCH (Secondary Synchronization Channel) signal that has the same shape as the PSCH signal except that packets of 256 chips transmitted are modulated by known information. FIG. 3 shows the order of the different synchronization steps of the UMTS HS-DPA supplementary link at chip, slot and frame levels respectively.
On the other hand, synchronization of the OFDM HS-DPA supplementary link is now more difficult because, unlike the UMTS HS-DPA supplementary link, the OFDM HS-DPA supplementary link is not intimately linked to the UMTS system. According to the current technique, the OFDM HS-DPA supplementary link can be synchronized at chip level using the delay interval that represents a part of the OFDM symbol (the last part). This synchronization at chip level is obtained by a simple conventional self-correlation on the received OFDM HS-DPA signal. But, once this synchronization has been done at chip level, it is impossible to know the beginning of the sub-frames and frames because the OFDM signal specified in the HS-DPA frame does not contain the PSCH and SSCH signals necessary for synchronizations at sub-frame and frame levels. According to the existing technique, synchronization of the OFDM HS-DPA supplementary link at sub-frame level is specific to the OFDM system and is based on insertion of signals specific to synchronization at sub-frame level. The major disadvantage of current art is that the above-mentioned specific signals increase the load of the OFDM HS-DPA supplementary link.